This invention relates to electrical fuses and more particularly to a composite fuse having two dissimilar fuse metals in series.
The prior art is replete with a host of patents describing various types of fuses for various purposes. As is well known, a fuse is an over current protective device with a circuit operating fusible part that is heated and severed by the passage of an over current through it. In regard to the structure of such fuses, they employ various types of fuse links. A fuse link may be a replaceable part or assembly which is comprised principally of a conducting element which element may be replaced after each circuit interruption to restore the fuse to the operating condition. The links of such fuses normally comprise a conductive metal, as for example copper, silver, aluminum, as well as other metals or various combinations or alloys of the above.
The prior art describes various materials which are employed in fuse links and specific assemblies of certain links as well as fuse constructions may be had by referring to U.S. Pat. No. 4,308,515 entitled FUSE APPARATUS FOR HIGH ELECTRIC CURRENTS, issued on Dec. 29, 1981 to W. J. Rooney, et al and assigned to the assignee herein.
As is well known, copper is a widely employed material which is used in many fuse links. Copper is a relatively good conductor, but as is understood, there are other elements which are better conductors. In regard to this silver is a better conductor than copper and the amount of energy required for heating silver is greater.
In regard to a link composed of silver one can usually provide a thinner link when employing silver than when employing copper for the same operating characteristics. When employing silver in a fuse, the formation of silver oxide enables such links to exhibit superior arc quenching capabilities during fuse operation. Essentially, when a fuse is severed, based on normal inductance in circuit operation, the current through the fuse is not interrupted instantaneously, and thus an arc is produced across the fuse elements. The fast interuption of such an arc may be damaging to the circuit components which are protected by a fuse.* In a fuse employing silver, the silver oxide formed acts to conduct the high voltage, and hence the arc is dissipated more rapidly than for example in a fuse employing a copper link. FNT because of the high voltages that can be produced by such actions.
On the other hand, copper has more resistance than silver for the same amount of material and a copper link would heat up faster. In regard to this heat the copper link stores more energy, and based on the heat, the copper link will sever more rapidly than a silver link but will produce greater arcing. It is, of course, a desire during fuse operation to produce a fuse which is capable of interrupting a short circuit current rapidly while producing a relatively low energy arc in order to further protect the circuit.
It is, therefore, an objective of the present invention to provide a composite fuse link employing a first section of copper and a second section of silver both of which are employed in series to enable the composite link to exhibit a rapid fuse operation with a low arcing capability.